Modulated carrier wave transmitter



April 13, 1937. w. s. L. TRINGI IAM 2,076,685

MODULATED CARRIER WAVE TRANSMITTER Filed Nov. 16, 1934 POWER MT +F 5 AZ6 www EDT E ATTORNEY Patented Apr. 13, 1937 UNITED STATES l? I IMODULATED CARRIER WAVE TRANSMITTER William Stuart Leader Tringham,Surrey, England, assignor to Radio Corporation of America, a corporationof Delaware Application November 16, 1934, Serial No. 753,291 In GreatBritain November 17, 1933 7 Claims.

This invention relates to modulated carrier Wave transmitters andapparatus for use therewith. More specifically the invention relates toWhat may be termed modulation monitoring arrangements for use inconnection with radio and like transmitters and whereby the correctnessof the modulation at diiierent stages of the transmitter may be checkedfrom time to time and during operation of the said transmitter.

According to this invention a modulation monitoring installation, foruse for the purpose specifled, comprises a cathode ray tube, means forderiving from a plurality of stages of the transmitter to be monitored,potentials which should be like functions of the degree of modulation,and

switching means or its equivalent whereby any of the potentials soderived may be applied to deflect the ray in the cathode ray tube, thedeflection of the ray thus obtained serving as an informative indicationof the correctness or otherwise of operation of the transmitter.

Fig. 1 is a diagrammatic showing of the apparatus used in carrying theinvention, and Fig. 2 is a modification thereof.

One method of carrying out the invention will now be described withreference to Figure 1 of the accompanying diagrammatic drawing saidmethod relating to the primary intended application of the inventionwhich is to amplitude modulation television transmitters of the kindwherein modulation is effected at the grid circuit of a high frequencyamplifier stage.

In a practically convenient and satisfactory form of televisiontransmitter, the said transmitter is modulated on the grid circuit ofthe penultimate high frequency stage, in such manner that the carriercurrent in the output circuit of that stage should be at any instantdirectly proportional to the instantaneous brightness of the picture tobe transmitted.

' Thus a certain predetermined low carrier output level corresponds to ablack spot upon the picture and a high carrier leve1namely the peak ormaximum output which the transmitter can give practically withoutdistortion-corresponds to a spot of maximum whiteness or brightness. Incases where the synchronizing signals are transmitted as modulations ofthe same carrier wave-the usual casethe said synchronizing signals maybe transmitted, as is well known per Se, by modulating the carrier toreduce the carrier level below the minimum value corresponding to blackspots upon the picture.

In such a transmitter it is very desirable to be able from time to timeto observe instantaneously the levels of modulation at difierent partsof the transmitter installation as a whole, and in particular it is verydesirable to be able to compare wave shapes at (1) the input to theradio transmitter (2) the output of the modulated amplifier stage, and(3) the output of a. power amplifier stage which usually succeeds themodulated amplifier stage.

In applying the present invention to provide a monitoring installationfor use in connection with a transmitter as just described, there isprovided a switch device SW having [2 contacts arranged in a circle,these contactsbeing numbered in the drawing in clockwise direction fromI to [2.

Contacts l, 2 and 3 are connected together and earthed at E. Contacts 6,I and 8 are connected together and through a high frequency choke CH1 tothe gun or anode point to which .is connected one deflecting plate of acathode ray tube of ordinary oscillograph type such as shown by PatentNo. 1,592,274 to N. Kipping, issued July 13, 1926, and entitledEmployment of cathode ray oscillographs. Contacts 9, l0 and II are alsoconnected together and through a choke CH2 to the deflecting plateterminal of the cathode ray tube. The ends of the chokes CH1 CH2 remotefrom the switch contacts are connected together through a resistance R,the ends of which in consequence constitute the input terminal IT of acathode ray oscillograph which is the indicating device for the wholeinstallation. It is understood that the other pair of deflecting platesare connected to a sweep circuit as is illustrated in the above citedpatent to Kipping. To avoid unwanted pick up of high frequency currentsthe choke CH1 and CH2 should be connected in circuit as close to thesaid oscillograph as possible. The switch contacts are uniformly spacedas shown so that contact l is diametrically opposite contact 1 andcontact 2 is diametrically opposite co'ntactB, and so on. The switch hastwo diametrically arranged rotatable bridge pieces A1 and A2 each ofwhich can connect a desired pair of diametrically opposed contactstogether. Switch contact I2 is connected to the positive terminal of abias bat-- tery B and a movable tapping point T1 thereon is connected toa movable tapping point T2 upon a potentiometer resistance M whichconstitutes the modulation resistance in the grid circuit of the highfrequency amplifier valves V1 V2 at which modulation is accomplished.Resistance M is earthed at one end and connected to the grid point ofthe parallel connected valves V1 V2 at the other. Switch contact 4 isconnected through a high frequency choke CH3 to the electrical centre ofa coil C1 whose ends are connected to the anodes P1 P2 of a full waverectifier D1 having two anodes and a cathode K1, as in the usual way,the cathode being earthed. This coil C1 is a coupling coil and islooselycoupled to the anode circuit of the power amplifier 2| followingthe stage V1 V2 at whichmodulationis effected i. e. the modulatedamplifier stage. switch contact 5 is similarly connected through a highfrequency choke CH4 to the electrical centre of a second coupling coilC2 whose ends are con-- nected to the anodes P3 P4 of a full waverectifier D2 having an earthed cathode K2. The coupling coil C2 isloosely coupled to theanode' circuit of the high frequency amplifier V1V2 at which mod-- ulation is effected. Filament heating currents for thetwo full wave rectifiers are taken through suitable resistances from themain low tension 0 supply for the transmitter as a whole.

point T are so adjusted that with the minimum amount of grid bias on thegrid of'the modulation stage V1 V2 which will give new carrier levelfrom the transmitter, the deflection of the cathode ray tube when its,inputterminals receive potential through contact I2 is, zero and sothat with a given carrier level the deflection of the cathode ray tubeis the same whichever of the three supplies is fed thereto by theswitch,i. e. whether the input to the cathodev ray tube is derivedfrom contactsI2, 4 or 5. The two diametrical contact bridges A1 A2 of the switcharearranged to be rotated together by a single handle and to be at rightangles to oneanother so that, for example, when contacts 6 and I2 arebridged, contacts 3 and 9 are also bridged. It

will be noted that with the arrangement described the sense of thevoltages which are applicable to the cathode ray tube from the threediiferent sources, is such that the deflection will always be in thesame direction.

x The value of the resistance R. across the input terminals of thecathode ray oscillograph should be made sufficiently low to ensure thatthe capacity 'of the various inputs is negligible at all modulationfrequencies. If thisbe done theapparatusmay be employed for frequencyresponse measurements.

-In the modification shown in Figure 2 the coupling between the anodecircuit of the modulated amplifier stage. V1 V2 and thegridcircuit ofthe succeeding power amplifier stage (not shown) is effected through aconcentric feeder 00, IC, whose outer tubular conductor 0C is earthed.(Generally a similar method of coupling will be employed between thepower amplifier and the transmitting aerial). With. this method ofcoupling it is convenient to dispense with the coupling coils- C1 C2 ofFigure 1, and,

6 instead of employing inductive coupling to the rectifier D1 D2, totake the modulated high fre.

quency energy from the appropriate anode circuits via the concentricfeeders connected thereto and suitable small condensers. The circuitarrangement is shown in detail in Figure 2 for The remaining therectifier associated with the anode circuit of the stage V1 V2. As willbe seen this rectifier D2 has an earthed cathode K2. and has its anodePa connectedthrough achoke CH4 to the contact 5 and also through thecoupling condensers F1'Fz to the inner conductor 10. The conductors 10,0C, are coupled through condensers F3 F and coil C3 to the anode circuitof stage V1 V2. The other rectifier D1 has its cathode K1 earthed andits anode P1 connected through chokeCHa to contact 4, said anode beingalso connected through. coupling condensers F1, F2 to the innerconductor of the tubular concentric feeder (not shown) associated withthe anode circuit of the power amplifier stage (not shown). S1 S: arescreens. The use of coupling condensers for taking. modulated highfrequency energy from the concentric feeders is not necessary and othercoupling expedients may be resorted to; for example, resistances may beused although such use islnot. preferred owing totheheating effects inresistance. Again, it is possible 'to use a.combi-. nation of couplingcondenser on the live side,

following'by avariable resistance. This arrangea,

ment is preferable to thetuse of coupling resistances simply, but thereis little to choose between 1 it and the use of coupling condensers asillustrated.

Preferably the oscillographlused in outthis invention .is mountedon ornearthe' same control board or ,table, as the, main controls. of thetransmitter to ,be monitored or checked, in order that thesaidoscillograph may,

bdobserved. while operating, the transmitter ,c0n-. v trols.

. The, oscillograph the carry through or degree of unbalance of highfrequency energy when adjusting theneu-.

tralizing. reactances of a neutralized highfrer quency valve amplifieremployed in the transmit- That is to say that if the amplifier is unter. balanced as regards to neutralization, radio. frequency energy willbepassed throughthe grid further it canreadily vbeascertained whatper-v:can. be used for measuring,

The deflection of the ray will bean centage of the normal carrierstrength the carry through is.

Havingnow described my inventionQwhat I claim is: I p

.1. A modulation monitoring installation for use with a modulated radiotransmitter whereinis provided a;plurality of stages comprising acathode raytube oscillograph, means for deriving from a plurality ofstages of the transmitter to be monitored potentialswhich are -=prede-rtermined functions of the degree of modulation and a single switchingmeans whereby only one.

2. A modulation monitoring installation for use with amodulated radiotransmitter wherein is provided a plurality of stages comprising a.

cathode ray tube oscillograph, means for deriv be, monitored, means forrectifying said energy,

means for deriving from astage succeeding said modulatedamplifier stagemodulated high frequency energy, meansv for rectifying said last.

mentioned energy, means for deriving from an impedance in thetransmitter and wherein modulation energy flows, potentialscorresponding to said modulation energy, and a single switching meansfor applying at any instant only one of said mentioned potentials oreither of the rectified outputs to the input terminals of a cathode rayoscillograph.

3. A modulation monitoring installation for use with a modulated radiotransmitter wherein is provided a plurality of stages comprising acathode ray tube oscillograph, connections between one input terminal ofsaid oscillograph and three adjacent contacts of a rotary switch havingtwelve equally spaced contacts arranged in a circle said switch alsohaving two mutually perpendicular bridge arms adapted to be movedtogether each bridge arm being adapted to connect together diametricallyopposite contacts of the switch a connection between three furtheradjacent contacts of the switch and the other input terminal of theoscillograph a connection between an impedance of the transmittermonitored, through which modulation energy flows, and a seventh contactof the switch, a connection between an eighth contact of the switch anda rectifier connected and arranged to rectify modulated high frequencyenergy derived from the anode circuit of a modulated amplifier in saidtransmitter, a connection between a ninth contact of the switch and arectifier connected and arranged to modify modulated high frequencyenergy derived from the anode circuit of a stage succeeding saidmodulated amplifier stage, the remaining three.

switch contacts being connected together and earthed, the wholearrangement being such that by rotating the two bridge arms together todifferent positions either voltages from the modulation impedance orvoltages from either of the rectifiers may be applied to deflect the raywithin the cathode ray tube.

4. An installation as claimed in claim 3 and wherein the rectifiers areenergized through inductive coupling circuits are of the full wave type.

5. An installation as claimed in claim 3 and wherein the rectifiers areenergized through capacitative coupling means connected to concentricfeeder coupling devices in the transmitter.

6. An installation as claimed in claim 3 and comprising chokes insertedin the leads to the input terminals of the cathode ray tube oscillographsaid chokes being adjacent said terminals.

7. An installation as claimed in claim 3, and

WILLIAM STUART LEADER TRINGHAM.

